“I’ll have what she’s having.” Sound familiar? It might if you dine often with your slim friends, suggests a recent study in the Journal of Consumer Research. Female college students were invited to watch a movie and offered munchies (granola and M&Ms) to snack on. Students either watched the film alongside a researcher who was thin or with the same researcher wearing a fat suit to make her look obese. Surprisingly, participants ended up eating almost twice as much when the researcher was thin than when she was obese.

“If we have a thin friend who ‘pigs out,’ we think, ‘If she can eat like that and stay thin, so can I!’” says study author Brent McFerran, Ph.D. There are several reasons why this may not be true, he says: metabolism, body size, genetics, activity level, age and gender all affect how much one can consume without gaining weight. Those factors aside, adds McFerran, consider that your friend maybe didn’t eat a large breakfast or for her this is a rare treat.